This application claims priority of Disclosure Document No. 467,710, filed Jan. 18, 2000.
1. Field of the Invention
This invention generally relates to radio navigation techniques using Global Positioning System (GPS) signals. More particularly, the present invention relates to the augmentation of conventional GPS receivers with attitude determination capability that can operate in an interference-rich environment by using adaptive nulling and constrained beamforming monopulse techniques.
2. Description of the Prior Art
GPS is one of the most advanced satellite radio navigation systems, maintained by the government of the United States of America. GPS radio navigation relies upon a constellation of twenty-four active satellites in six different orbits around the globe. GPS position fixes are obtained by measuring the propagation delay times of GPS radio signals broadcast by the orbiting GPS satellites. Normally, a user must receive the signals from at least four GPS satellites in order to solve the variables of longitude, latitude, and altitude, as well as timing error that are needed to precisely determine location and time. As its name implies, GPS is a positioning device. In order to see at least four satellites simultaneously on or near the earth, an antenna with a hemispheric reception gain pattern is typically used. See B. W. Parkinson and J. J. Spilker Jr. (eds.), Global Positioning System: Theory and Applications, Published by the American Institute of Aeronautics and Astronautics, Inc., 1996 and E. D. Kaplan (ed.), Understanding GPS: Principles and Applications, Artech House Publishers, 1996 for a detailed description of GPS and its operations and applications.
GPS phase measurements turn out to be of very good quality, and this has prompted a widespread use of interferometric techniques. More successful use is in surveying applications with double-difference kinematic GPS. GPS interferometric attitude determination has also seen some applications. GPS interferometric attitude determination requires at least three antennas at the tips of two non-parallel baselines separated by several wavelengths. For a given level of phase measurement quality, the attitude estimation accuracy of the technique is proportional to the baseline length. However, continuous outputting of attitude information hinges on reliable resolution of phase integer ambiguity and repair of cycle slips after their detection. Both techniques are computationally very demanding in real time. Furthermore, most interferometric configurations for GPS attitude determination do not include any provision against the presence of interference such as jamming and/or multipath.
Recently, array antennas have been introduced mostly in military applications for jam-suppression and signal to noise ratio (SNR) enhancement. It is therefore an object of this invention to utilize array antennas for GPS attitude determination in addition to conventional positioning and timing with an improved capability of operating under interference.
However, a straightforward combination of GPS interferometry and multiple antenna arrays for attitude determination, assuming it would work, is not cost-effective. Each antenna array with an anti-jam spatial filter may serve as a jam-free antenna. However, the spatial filters associated with these antenna arrays have to affect the signal in a very similar manner in order to preserve the gain and phase relationships. This is too costly and cumbersome. More critically, anti-jamming manipulation of GPS signals would introduce adverse effects on kinematic carrier phase attitude determination that are difficult to constrain or minimize. That is, the very same carrier phase measurements needed for attitude estimation have been adjusted in gain and/or phase by an anti-jam spatial filter in antenna electronics to attenuate jammers. Even if a jamming signal is totally suppressed from the output of an anti-jam spatial filter that inputs from an antenna array, the jamming signal still corrupts individual array antenna elements. As a result, the spatially filtered antenna array signal, as a whole, can be used for positioning but not for attitude determination. This is because of unpredictable phase distortion. Similarly, individual antenna elements cannot be used for attitude determination, as in a conventionally interferometric manner, because these antenna elements still contain jamming signals. It is therefore an object of this invention to seek other rather interferometric techniques for GPS attitude determination under interference.
Quinn and Crassidis (see D. A. Quinn and J. C. Crassidis, xe2x80x9cGPS xe2x80x98Compound Eyexe2x80x99 Attitude Sensor,xe2x80x9d NASA Tech Briefs, May 1999) outlines an approach, different from interferometry, to attitude determination using GPS signals. In their design, multiple directional antennas are mounted on a convex hemispherical surface with polyhedral arrangement. Each antenna is thus aimed to receive GPS signals from a certain field of view, called a visualization cone. Their idea is to use the special GPS antennas arranged in a xe2x80x9ccompound eyexe2x80x9d as star trackers and the GPS satellites as well-known, well-behaved stars. As GPS satellites pass through the various fields of view, attitude can be determined in a manner identical to what has been employed by standard star trackers for many years. Compared to interferometric GPS, the xe2x80x9ccompound eyexe2x80x9d is easy to get a first fix to attitude and its accuracy is not limited by structure size. Because it does not require carrier phase or Doppler shift estimation, it is much less sensitive to Doppler and multipath effects as well as line biases. The antenna geometry provides maximum sky coverage without the need for self-survey calibrations. However, the attitude estimation accuracy of the xe2x80x9ccompound eyexe2x80x9d depends upon the number of antennas and, more critically, upon the precision at which individual antenna patterns can be physically shaped, oriented, and mounted. The latter is imaginably associated with substantial cost of manufacturing and installation particularly when it is small.
Alternative to the interferometric GPS and different from the special antenna system of a xe2x80x9ccompound eyexe2x80x9d attitude sensor, the present invention solves the directions of arrival of GPS incident signals for attitude determination using GPS monopulse. Monopulse is a mature technique widely used in surveillance and tracking radar. See, for example, D. K. Barton, Modern Radar Systems Analysis, Artech House, 1988, M. Sherman, Monopulse Principles and Techniques, Artech House, 1988, and I. Leanov and K. I. Fomichev, Monopulse Radar, Artech House, 1986. In simple terms, monopulse angular measurement is based upon distinct angular responses of directional antennas (i.e., with finite beamwidth) to signals incident from different directions. For each angular dimension (azimuth or elevation), two squint antenna patterns are formed with an angular displacement typical half of the beamwidth, from which a pair of sum and difference beams are generated. The ratio of the sum over difference beam responses provides the measurement of an angular offset of the incident wavefront off the array boresight. The squint beams can be generated either physically with directional antennas or with omnidirectional antenna arrays electronically or digitally in a digital beamforming process.
In general, angular accuracy, in terms of estimation error, is inversely proportional to aperture size. For an array antenna with a small number of elements, the GPS monopulse technique may provide an angular solution less accurate than that of the GPS interferometry technique. However, the latter can only do so if good phase measurements are available as previously analyzed. Besides, it may suffer from the problem of carrier cycle integer ambiguity, further complicated by cycle slip. Its solution cannot be made available until code and carrier tracking have been achieved and maintained in the receiver. In comparison, the GPS monopulse technique is faster, more robust, and less vulnerable than the interferometric counterpart. In one application, the GPS monopulse technique may be used to provide a quick xe2x80x9ccoarsexe2x80x9d attitude solution to initialize a slow xe2x80x9cfinerxe2x80x9d interferometric attitude solution and to back it up when the interferometric solution becomes unavailable due to cycle slips or other outages.
Compared to omnidirectional or hemispheric antennas used in conventional GPS receivers, array antennas provide the directionality by spatial filtering, with which antenna patterns or beams can be electronically and/or digitally synthesized and steered in adaptation to the changing environment. One example is to maximize the reception in the direction of desired signals, while minimizing the reception in the direction of unwanted signals. Since most interference sources are unknown, a priori and may be moving, their angular locations have to be learned from data adaptively. To use monopulse under interference, nulls toward interference have to be placed simultaneously in both the sum and difference beams in the same directions. Further, the nulling in the sum and difference beams has to be constrained so that it will not distort the monopulse ratio slope in order to maintain the accuracy of angular measurement. These requirements have naturally lead to a constrained beamforming adaptive monopulse.
Most constrained beamforming, adaptive nulling, and digital monopulse techniques have originated from and thus been developed for radar applications. The radar signal characteristics, system architecture, operating mode, application environment, and information to extract are quite different from those in GPS applications of interest to the present invention. It is therefore an object of this invention to adapt and improve on constrained beamforming adaptive monopulse techniques for GPS attitude determination.
A conventional GPS receiver is capable of providing the position and time information. When augmented with the attitude sensing capability of the present invention, it will provide the time, translation (position and velocity), and rotation (attitude and angular rates) seven degrees of freedom (7DOF) information. It becomes fully capable of navigation. However, the navigation capability offered by the present invention differs from an inertial navigation system (INS) made up of accelerometers and gyroscopes (see, K. R. Britting, Inertial Navigation Systems Analysis, Wiley-Interscience, 1971) in four aspects. First, the GPS navigation system according to the present invention can provide a very accurate time information that an INS cannot. Second, most inertial sensors such as gyros and accelerometers are xe2x80x9cintervalxe2x80x9d sensors in that they measure incremental changes in angle and velocity, respectively. These measurements have to be integrated over time once and twice, respectively, to produce attitude and position. By consequence, how to initialize the inertial integration becomes a critical problem. The GPS navigation system of the present invention is a xe2x80x9cpointxe2x80x9d sensor in that it directly measures the position and attitude. Third, due to accumulation by integration, the inertial navigation solution experiences an unbounded error growth, while the GPS solution exhibits a uniform error around the global and over time. Fourth, both INS and GPS are passive system but the former is self-contained in that it does not need any external information except for initialization. This is advantageous and sometimes necessary for some applications. As a result, the two systems may be used in a complementary manner. Being a point sensor also suggests an interesting mode of operation in which the GPS navigation system of the present invention intermittently operates on incident signals of short segment just enough for a point solution. The present invention thus has the potential to meet the navigational needs of special cases where continuous operation is impossible due to signal blockage or the sensor is exposed to the GPS signals only for a short period of time. Intermittent operation may just be desired to conserve energy or inexpensive processors used in the sensor can only handle the computational throughput for a snapshot solution.
Ince and Smith (see M. D. Ince and R. H. Smith, Method and System for Attitude Sensing Using Monopulse GPS Processing, U.S. Pat. No. 6,018,315, Jan. 25, 2000) disclose a GPS attitude sensor design having the potential to replace magnetometers, sun sensors, and/or horizon sensors for spacecraft attitude control. But their invention is distinguished from the present invention in five aspects. First, Ince""s and Smith""s invention is basically an analog design with radio frequency (RF) sum and difference circuitry. The present invention presents a digital design with the sum and difference beams formed in the baseband. Second, they utilize a pair of antennas with fixed patterns toward one satellite and in order to steer the beams for alignment, the entire platform has to be physically rotated. The present invention employs an array antenna that can digitally form beams and steer them toward any GPS satellite visible. Third, due to the beamwidth conflicts (i.e., narrow beams for monopulse angular measurement and omnidirectional for regular GPS receiver), two types of antennas have be configured in their design, while an array antenna is the only one used in the present invention. Fourth, Ince and Smith do not incorporate any interference suppression capability or ability to maintain monopulse accuracy under interference in their design. Fifth, the monopulse processor in their invention supplies an angular offset estimate to the attitude control unit for a single-axis attitude adjustment (e.g., tilt correction), rather than providing the full three-axis attitude solution (e.g., roll, pitch, and yaw) relative to a pre-defined coordinate system for the navigation purpose as in the present invention.
A need therefore exits for an augmentation of conventional GPS receivers with attitude determination capability that can operate in an interference-rich environment and that can service attitude control, navigation, and interference-locating purposes among others. This need is met by the present invention as described and claimed below.
The present invention is a signal processing method and system that can make a regular GPS receiver equipped with an array antenna capable of determining its own attitude in addition to regular timing and positioning under jamming conditions. The augmented signal processing system comprises a cascaded parallel architecture inserted between the array antenna and the regular GPS receiver. A blind adaptive nulling processor is cascaded ahead of the regular GPS receiver so that the GPS receiver can operate on the jamming-suppressed signals in a normal manner. A monopulse angular measurement unit in series with an attitude determination unit interacts in parallel with the blind adaptive nulling processor and the regular GPS receiver. Constrained beamforming adaptive monopulse is used to simultaneously place nulls in both the sum and difference beams toward jammers while maintaining the monopulse ratio for accurate angular measurement. A regular GPS receiver is modified so as to provide information necessary for monopulse angular measurement and integrated attitude determination. In this way, the augmented GPS receiver is capable of providing rotational information (three-axis attitude and angular rates) in addition to regular time and translation (position and velocity) information, hence a 7DOF receiver. The augmented GPS receiver can also be used as a self-contained device to locate the sources of interference among other applications.